Field of the Invention
The invention relates to a method for coating a component with a thermal barrier coating, in which the component is disposed in a coating chamber. The invention further relates to a coating device for producing the thermal insulation on a component, having a heating device for heating the component and a vacuum-generating device.
U.S. Pat. No. 5,238,752 describes a method for producing a thermal insulation layer system having an intermetallic bond coating for a small metallic component, in particular an aircraft engine blade having a length dimension of about 5 cm. The actual thermal insulation layer, made of zirconium oxide, is deposited on the component through the use of an electron-beam PVD (physical vapor deposition) method, with zirconium oxide and yttrium oxide being evaporated from a metal-oxide body using an electron gun. The method is carried out in a device in which the component is heated to a temperature of about 950.degree. C. to 1000.degree. C., before the coating process. A heater is provided in the device for heating the component from above, if appropriate, in addition to heating due to the zirconium oxide to be deposited, and radiation emerging from the surface of the evaporating ceramic body. In order to deposit the zirconium oxide, a vacuum of about 7.times.10.sup.-3 Pa (7.times.10.sup.-5 mbar) is generated in the device, and a deposition rate of about 100 .mu.m/h to 250 .mu.m/h is achieved with the electron gun. With that operating set-up, the intention is for a thermal barrier coating of zirconium oxide with a columnar microstructure to be formed on the all-metallic component.
U.S. Pat. No. 4,676,994 describes a method for depositing a ceramic coating on a substrate that has a ceramic surface in that case, in which a first ceramic material is heated in vacuo by an electron beam in such a way that it forms a sub-stoichiometric ceramic fluid. The substrate is heated in vacuo to a temperature of above 900.degree. C. and the first ceramic material is sub-stoichiometrically vacuum-evaporated thereon in order to form a dense ceramic layer. A layer of a ceramic with columnar orientation is deposited on that dense ceramic layer. During the vapor deposition of the sub-stoichiometric ceramic for producing the dense ceramic layer, the substrate temperature is maintained at a value between 900.degree. C. and 1200.degree. C., and the vacuum pressure is preferably below 13.times.10.sup.-3 Pa. During the coating of the dense ceramic layer with the columnar ceramic layer there is an oxygen partial pressure between 60.times.10.sup.-3 Pa to 0.27 Pa, and the combined partial pressure of other gases is less than 10% of the total pressure. The method was, for example, carried out on a gas-turbine blade having a maximum length of 10 cm.
German Published, Non-Prosecuted Patent Application DE 195 22 331 A1 describes a cathodic arc-evaporation method for the production, in particular, of metal oxide layers and layers of alloy oxides. In that case, a target is evaporated in an atmosphere including oxygen through the use of cathodic arc evaporation, so that the oxide of the metal alloy is essentially in a single crystallographic phase. The oxygen partial pressure during the coating process is observed, and deviations from a set-point partial pressure are minimized by setting at least one of the parameters: oxygen mass flow, arcing voltage or field strength of a magnetic field essentially perpendicular to the target surface. As an alternative, the arcing voltage is observed, and deviations from a set-arcing voltage are minimized by setting at least one of the parameters: oxygen mass flow or the magnetic field. As a third alternative possibility, the frequency spectrum of the discharge current is observed and deviations of characteristic components of the spectrum from set-point characteristics are minimized by setting the arcing voltage, the oxygen mass flow or the magnetic field. The above-mentioned parameters are preferably set automatically by a control loop for the working point of the process. In that case the method is exclusively intended for applying a coating of an aluminum oxide or a chromium oxide.
German Patent DD 299 902 A7 describes a method for operating a plasma arc of a hollow-cathode evaporator source, in which an emergency switch-off is substantially avoided in the event of operationally induced non-uniformities of the plasma arc occurring. In that case, a logic signal is formed as a function of a voltage drop between the hollow cathode and an evaporator crucible, or of a discharge current which is set up from the potential of the vacuum chamber through a resistor to the anode. Measures are taken to avoid an emergency switch-off depending on the value of that logic signal.
In an article entitled "Zirconia Thin Film Deposition on Silicon by Reactive Gas Flow Sputtering: The Influence of Low Energy Particle Bombardment" by T. Jung and A. Westphal, in Material Science and Engineering, A 140, 191, pages 528 to 533, the so-called reactive gas flow sputtering method is put forward for the production of a zirconium oxide layer on a semi-conductive substrate, in particular based on silicon. According to that sputtering method, an inert gas, in particular argon, is fed through a hollow cathode, in the interior of which an anode is disposed, so that ionization of the argon atoms takes place. The atoms strike the metallic cathode, formed of zirconium, as a result of which metal atoms and/or metal clusters are detached therefrom and are entrained with the inert gas flow. Outside the cathode, oxygen is supplied with a partial pressure of 10 Pa to about 10.sup.-4 Pa, for full oxidation of the metallic zirconium. The semiconductor substrate is fastened in a stainless steel holder that can be heated to 800.degree. C., and is heated to a temperature of about 400.degree. C. The method is carried out in a coating chamber which is evacuated to a high vacuum of about 10.sup.-7 Pa. The deposition rate is about 0.9 .mu.m h.sup.-1.
An alternative structure of a hollow cathode for the reactive gas flow sputtering method is described in an article entitled "High Rated Deposition of Alumina Films by Reactive Gas Flow Sputtering" by T. Jung and A. Westphal, in Surface and Coatings Technology, 59, 1993, pages 171 to 176. The hollow cathode specified therein has a linear structure, insofar as zirconium plates are disposed next to one another in a housing. An inert gas flow can be fed through between each pair of neighboring plates, so that a plasma of inert gas atoms is formed between neighboring plates. Using the method, test-pieces of silicon, stainless steel and glass were coated with aluminum oxide. During the coating, the temperature of a test-piece was between 100.degree. C. and 200.degree. C. The pressure inside the coating chamber was about 10.sup.-8 Pa, and the amount of oxygen supplied was 4.5 cm.sup.3 min.sup.-1. The deposited aluminum layer had a thickness of from 0.5 .mu.m to 6.0 .mu.m and an essentially .gamma.-microstructure.